This invention relates generally to moisture characteristics of porous materials, such as soil, wood, concrete and the like. It relates more particularly to a method and apparatus for directly measuring the moisture characteristic curve of such a porous material by measuring the pore water tension within the body, while simultaneously recording the change in mass due to removal of water.
The behavior of extended porous bodies, such as soil, and more discrete porous bodies, such as lumber and concrete, can be understood and anticipated, to some extent, with knowledge of the bodies' moisture characteristic. The moisture characteristic relates, on the one hand, moisture content, and, on the other hand, induced liquid tension. For instance, it is common to represent the moisture characteristic of soil as a curve, relating soil water content by mass to soil water tension, measured in bars, or kPa. In the field of soil mechanics, the characteristic is called the Soil Moisture Characteristic ("SMC"). For purposes of generality, the characteristic will be referred to herein as simply the Moisture Characteristic ("MC"), indicative of the fact that porous bodies other than soil may also be analyzed with reference to their moisture characteristics. FIG. 1A shows a typical MC for a sample of glass spheres, rather than soil. The regular curve, identified by small circles, was generated using an embodiment of the invention, discussed below. The data points indicated by open squares were generated using a prior art pressure plate apparatus, after the specimen is maintained under a pressure for 24 hours. The data points indicated by solid diamonds were generated using a prior art pressure plate apparatus after observable outflow terminated.
FIG. 1B shows an SMC for a specimen of soil, generated using an embodiment of the invention, discussed below, having a curve that has several changes in slope and curvature resulting from the more complicated pore structure of the soil as compared to that of the glass beads.
Rather than "tension," this property of soil is sometimes referred to as "suction." The term "suction" is used in various ways in the literature. For a good review of these variations, see, Ridley, A. M., and Wray, W. K., "Suction measurement: A review of current theory and practices, Unsaturated Soils (Alonso & Delage (eds.)) pp. 1293-1322, AA Blakema, in 1.sup.st International Conference on Unsaturated Soils, Paris, France (1996), which is incorporated fully herein by reference.
As used herein, "suction" or "tension" refers to the "matric" suction, which is a measure of the energy resisting movement of a molecule of water into the soil matrix from a reference pool, under isothermal condition and at constant elevation and pressure.
Knowledge about the SMC is useful to agronomists for many tasks. It can aid determination of suitability for various types of plant growth, and when or how frequently to water. It may also be used to determine how contaminants in ground water, or soil will be transported throughout the soil, and to analyze the behavior of contaminant plumes.
Insight regarding the strength of soils may also be gained using the SMC.
Meteorologists may also use knowledge of the SMC to enable them to determine the heat transfer properties across the landscape, which influences weather patterns.
Foresters may also use the SMC to determine whether a forest is at risk for irreversible fire damage, as opposed to whether such a forest might receive benefits from periodic burning, to a certain degree.
Civil engineers may use the MC of concrete to assess its integrity. It is thought that water tension in concrete is related to the propensity to experience micro cracks, which are the progenitors of macro cracks and subsequent failures.
The presently used methods for establishing SMC are inadequate for several reasons. One method is according to an ASTM standard, and uses a porous pressure plate. The porous plate method is used to determine the moisture--suction relationship for coarse and medium grained soils. The technique is recommended for suctions between 0.1 and 1 ATM. The use of this method is outlined in ASTM D2325-68 and is shown schematically with reference to FIG. 2. A similar method for higher suctions (finer textured soils) is set forth at ASTM D3152, recommended for suction up to 15 ATM.
The ASTM method uses a pressure container 10, a fine pore porous ceramic plate 12, brass screen, rubber (neoprene) membrane, sample rings, and various tubing and spouts. Approximately twenty-five grams of soil 14 is placed into a retainer ring ten mm high by fifty mm in diameter. The specimen and saturated porous plate 12 are then placed into the pressure chamber 10, where the air pressure is regulated by an outside source 16. By raising the air pressure in the chamber 10 to the desired value, drainage of the soil pore water is initiated and the expelled water is removed into a graduated cylinder 20 from the system via an underlying brass screen and neoprene membrane. The chamber pressure is maintained until cessation of pore water flow into the graduated cylinder. At this point, the chamber pressure is released and a water content of the sample 14 is taken by measuring the mass, first wet, and then oven dried.
Since the pore water is free to drain to an outside sink that is open to the atmosphere, the difference in chamber pressure to atmospheric pressure is assumed to be equal to the matric suction. A major drawback of this prior art technique is that it takes several weeks, or even months to generate an SMC curve. This is because each measurement establishes only a single data point relating water content to imposed pressure. Further, it can take several days to collect each data point, because it takes a long time for the plate to saturate, and after that, a long time to come to equilibrium (i.e., for the water to cease flowing out of the chamber into the graduated cylinder 20).
Thus, for each point of tension, a new run of the experiment must be conducted. Typically, only a few data points (fewer than ten) can be generated during practical time periods. The ASTM technique has many additional drawbacks, in addition to the lengthy time. One relates to a certain level of detail in the true SMC curve. Typically, the true SMC curves have a variable fine scale shape as shown in FIG. 1B, which occurs at a scale that is too fine to be captured by the small number of data points sampled with the prior art method. What is needed is a substantially continuous test.
Further, each test (i.e., each pressure) is run on a different specimen of soil. Thus, if sample storage, or collection is not adequately controlled, different data points will actually refer to different soil conditions.
It is difficult to determine when the system has come to equilibrium (i.e., when the water has ceased to flow into the cylinder), and thus from one specimen to the next, there is no certainty that similar situations are being compared, especially if different operators have conducted the tests. One reason for this is believed to be, that while the air pressure is applied to the soil, the soil specimen becomes drier even if no additional fluid collects in the graduated cylinder. Thus, more than one mechanism would seem to be at play in the soil moisture reduction.
Further, when the pressure is relieved and the specimen is removed from the pressure chamber, water that is in the apparatus (membrane, tubing, etc.), returns to the specimen due to the momentary pressure differential. Thus, the subsequent wet mass measurement of the soil does not actually correspond to the imposed pressure. This problem is mentioned in the ASTM documentation. This may be thought of as an elastic rebound of the system.
Another drawback is that the prior art method does not directly measure tension. It imposes a positive pressure and makes an inference that the moisture content under tension would be the same.
Thus, regarding soil, and SMC, there is a significant need for a method and apparatus to measure SMC that can provide results within a few hours. Further, there is a need for a system that can generate an SMC that is continuous over a wide range of soil moisture contents and liquid tensions and that is sensitive to fine scale detail of such characteristics. There is also a great need for a system that provides reproducible results, and that performs all relevant tests on the same specimen, rather than upon different specimens that must be carefully processed to minimize specimen geometry differences. There is also a need for a method and apparatus that directly measures the tension in the pore liquid, and that does not experience elastic rebound.
Thus, the several objects of the invention include to enable generating moisture characteristics, particularly for soil, within a practically short period of time, on the order of hours or a few days, rather than weeks. Another object is to enable such MC generation reproducibly, without regard to specimen size, geometry or ambient humidity, and to enable generation of an entire MC curve, over various moisture contents and tensions, using the same specimen. Another object of the invention is to generate such MC curves substantially continuously over both moisture content and tension ranges, with a level of detail that is much finer than is now available. It is a further object to provide a system that measures pore tension directly.